As a friction welding method, PTL 1 discloses a technique for welding metal materials that involves rotating at least one of a pair of metal materials to generate frictional heat and soften the metal materials and stirring the softened portion to produce plastic flow.
Since this technique involves rotating metal materials to be welded, there are limitations on the shape and the dimensions of metal materials to be welded.
PTL 2 discloses a method (friction stir welding method) for continuously welding workpieces in the longitudinal direction using heat generated between the workpieces and a tool and plastic flow by causing the tool to penetrate into a non-welded part of the workpieces and moving the tool while the tool is rotated. The tool is made of a material substantially harder than the workpieces.
The friction welding method disclosed in PTL 1 involves welding workpieces with frictional heat between the workpieces while the workpieces are rotated. The friction stir welding method disclosed in PTL 2 involves welding the workpieces by moving the tool with the tool being rotated while the workpieces are fixed. Therefore, this method has an advantage of continuous solid-state welding of workpieces in the longitudinal direction, the workpieces being substantially infinitely long in the welding direction. Since this method is associated with solid-state welding using plastic flow of metal produced by frictional heat between the rotary tool and the workpieces, the workpieces can be welded together without the need to melt the welded part. Furthermore, this method has many advantages of, for example, less deformation after welding because of low heating temperature, fewer defects because of the fact that the welded part is not melted, and no necessity for a filler metal.
The friction stir welding method has a wide range of applications in the fields pertaining to aircraft, watercraft, railed vehicles, and motor vehicles, and other fields as a method for welding low-melting-point metal materials typified by aluminum alloys and magnesium alloys. The reason for this is that an arc welding process known in the art is unlikely to provide the welded part of such low-melting-point metal materials with satisfactory properties, and the use of the friction stir welding method improves productivity and the quality of the welded part.
The use of the friction stir welding method for structural steel mainly serving as a material for structures such as buildings, watercraft, heavy machines, pipelines, and motor vehicles avoids solidification cracking and hydrogen cracking, which are problematic in fusion welding known in the art, and also reduces microstructural changes in steel materials, which improves joint performance. The friction stir welding method also has an anticipated advantage of no necessity for a preparation step, such as diffusion welding, because stirring the joint interface with the rotary tool creates clean surfaces and enables contact between the clean surfaces. Consequently, the use of the friction stir welding method for structural steel has many anticipated advantages. However, challenges associated with welding workability, such as suppression of defect generation during welding and an increase in welding speed remain. These challenges hinder the friction stir welding method from being used for structural steel compared with low-melting-point metal materials.
In friction stir welding for structural steel, high abrasion resistance materials such as polycrystal boron nitride (PCBN) and silicon nitride (SiN4) are currently used for rotary tools as described in PTL 3 and PTL 4. However, since these ceramics are brittle, there are strict limitations on the thickness of steel sheets to be welded and the conditions for processing the steel sheets in order to eliminate or reduce damage to the rotary tool.
PTL 5 and PTL 6 disclose welding methods using an additional heating function in addition to frictional heat generated between the rotary tool and the workpieces in order to improve welding workability.
For example, PTL 5 discloses a heating apparatus for a friction stir welding method. The heating apparatus has a heating function realized by an induction heating device, which heats workpieces before and after welding to increase the welding speed and prevent or reduce cracking in the welded part.
PTL 6 discloses a friction stir welding apparatus having a heating function realized by a laser device, which partially heats workpieces just before welding to reduce changes in microstructure around the region heated by preheating and increase the welding speed.
The techniques disclosed in PTLs 5 and 6, however, do not provide sufficient welding workability because no attention is given to, for example, the surface temperature and the depth of the heated region of the workpieces during heating before welding. In addition, excessive heating changes the microstructure around the heated region, which may adversely affect the weld joint properties, particularly the weld joint strength. Therefore, a practical friction stir welding method that achieves sufficient strength and good welding workability and a device that realizes such a practical friction stir welding method have not yet been developed.